Simply to avoid boredom setting in, "perfect pleasure" wouldn't be "every moment exactly the same."
Even if there are "less pleasurable" moments, they don't need to become hell in absolute terms. In relative terms, they might; however they would become hell only if one sets a reference, a point of comparison, to define "below this, it's hell." Think heat: there's no such thing as anti-heat. What we call "colder" is simply less heat, and what we call "cold" is simply defined in relation to comfort for our body. Any temperature is positive above the absolute zero Kelvin (−273.15 °C, −459.67°F). You can always define an experience as bliss (I'm still alive after such car crash? wow! bliss!) or hell (I don't have the latest gadget? hell!); this has little to do with the absolute of the experience, which (like temperature) always exists above the "zero" of experience. There's no more anti-experience then there is anti-heat. When you're free of choosing the definition you use, you don't need to make "less pleasurable" into your hell.